This invention relates to electron beam tubes and more particularly to those for use in amplifying a high frequency signal, for example, for use in an inductive output tube (IOT) amplifier.
In an IOT, a high frequency input signal is applied in the region between a cathode and adjacent grid of an electron gun via an input cavity to modulate the electron beam generated at the cathode. An amplified high frequency output signal is coupled from an output cavity.
According to the invention, there is provided an electron beam tube for use in amplifying a high frequency signal comprising: a vacuum envelope containing an electron gun, the vacuum envelope including a ceramic cylinder having two straight-sided sections with a conical section therebetween them, the straight-sided sections forming part of r.f. choke means.
Use of the invention facilitates attaching the external part of a high frequency resonant cavity to the tube and enables good electrical connections to be easily made. The configuration of the ceramic cylinder enables the tube to be simply dropped into place in a cavity-forming part instead of, as in prior art arrangements, having to accurately fit the cavity components up against the vacuum envelope.
Preferably, metallisation is included on the outer surface of the straight-sided sections. The metallisation may cover only part or all of the outer surface of the straight-sided sections. The metallisation may cover a larger surface area than is necessary to adjoin the mating portion of the cavity-forming part to relax tolerancing requirements, as good electrical connection will still then be achieved if the axial alignment of the components to be joined is not completely correct. The metallisation on the outer surface may form by itself one plate of the rf choke means or may adjoin a metal component such that together they act as a plate. Alternatively, metallisation may be omitted and a metal component brazed to one or both straight-sided sections.
Preferably, the inner surface of the straight-sided sections of the ceramic cylinder are metallised. However, one or both of them may have a metal component brazed thereto.
The metallisation may also extend along the inner and/or outer surfaces of the ceramic cylinder adjacent the straight-sided sections. For example, the metallisation on the outer and/or inner straight-sided sections may continue to cover some of the conical section surface also. This leads to a longer if choke or chokes which may provide a further reduction in the amount of rf energy passing through the choke.
In a preferred embodiment, the ceramic cylinder has a wall of substantially uniform thickness along its length. However, in some embodiments, there may be a variation in thickness. The outer surface only of the ceramic cylinder may be conical, whilst its inner diameter remains constant along the length of the conical section, such that the wall thickness changes.
Preferably, the diameter of the conical section increases in the direction in which electrons of the electron beam travel.
In one advantageous arrangement, metallisation is included along the interior of the ceramic cylinder to make connections to an electrode or electrodes of the electron gun.
The electron beam tube may be included in an IOT amplifier but the invention may find applications in other types of device.
According to a feature of the invention, there is provided a high frequency cavity-forming part adapted for use with the electron beam tube in accordance with the invention.